1. Field of the Invention
The present invention relates to a vehicle gear transmission, and more particularly to a vehicle gear transmission having an overdrive gear stage.
2. Description of the Prior Art
A manually operated gear transmission for motor vehicles generally includes a countershaft adapted to be driven by an engine drive shaft and an output shaft which is located in parallel with the countershaft and adapted to be connected with vehicle driven shaft usually through a differential gear mechanism. Between the countershaft and the output shaft, there are provided a plurality sets of intermeshing gears of different gear ratios, one of the gear sets being selected to complete a power transmitting gear train of a desired gear ratio.
In conventional structures, so-called counter gears provided on the countershaft are mounted on the countershaft through splines so that they rotate together with the countershaft. The driven gears on the output shaft are rotatable with respect to the output shaft and mechanisms are provided for selectively connecting one of the gears to the output shaft. In connecting one of the gears to the output shaft, there usually is a difference in rotating speed between the output shaft and the one gear on the output shaft so that a synchronizing mechanism is provided for each gear on the output shaft. Usually, such synchronizing mechanism is provided between two adjacent gears on the output shaft so that one such mechanism can be used in common for the two gears at the opposite sides of the synchronizing mechanism. For example, in a gear transmission having an overdrive gear stage, a first synchronizer is provided between the first and second stage driven gears, a second synchronizer is provided between the third and fourth stage driven gears, and a third synchronizer is provided between the overdrive driven gear and the reverse stage driven gear. The arrangement is believed as being effective in making the structure simple.
In a gear transmission having an overdrive gear stage, the gear on the countershaft for the overdrive gear stage is of a large diameter in order to provide a desired gear ratio. In case where such a counter-overdrive gear of a large diameter is provided to rotate with the countershaft, there will be a substantial increase in the inertia of the countershaft. This will cause inconveniencies in gear shifting operations in that the rotating speed of the counter-overdrive gear has to be synchronized with that of the output shaft overcoming the inertia of the countershaft. Thus, the shifting operations are made difficult and the synchronizing mechanism is subjected to an excessive load.
In order to eliminate the problems, there is proposed, by Japanese patent application No. 58-152948 which has been filed on Aug. 22, 1983 and disclosed for public inspection under the disclosure No. 60-44640, to mount the counter-overdrive gear rotatably on the countershaft and instead to connect the overdrive gear with the output shaft to rotate therewith. The arrangement is considered as being advantageous in that the inertia of the countershaft can be decreased.
It should however be noted that the arrangement as proposed by the aforementioned Japanese patent application is disadvantageous in that an additional synchronizer has to be provided between the countershaft and the counter-overdrive gear on the countershaft. In order to make it possible to actuate the synchronizer between the countershaft and the counter-overdrive gear, the shift rod for the overdrive gear stage and the reverse gear stage has to be provided with a shift fork for actuating the synchronizer for the overdrive gear stage as well as a shift fork for actuating the synchronizer for the reverse gear stage. Thus, a complicated mechanism is required. It should further be noted that in a conventional arrangement, the shift rod is located above the gear mechanism whereas the countershaft is in a lower portion of the gear mechanism so that a shift fork of a substantial length has to be provided in order to actuate the synchronizer on the countershaft. This will cause a further problem because there is a possibility that the rigidity of the shift fork is decreased to an undesirable extent. In order to increase the rigidity of the shift fork, the size of the shift fork is undesirably increased.